It has been found that the molar entropy of vaporisation of globular-shaped molecules or those with effective spherical field symmetry can be uniquely expressed as a function of molecular number (sum of atomic numbers) only. The higher values for non-globular molecules are plausibly related (through an aspherical factor, θ, which has certain additive properties) to molecular shape and consequent molecular parallelism in the liquid state. The entropies of vaporisation of flexible molecules involve an additional term of magnitude proportional to the number of flexible links present. Expressions are developed, the use of which permits (a) estimation of latent heats of a great variety of materials with a reliability of better than 1% and (b) a reliable diagnosis of molecular association.